Computed tomography scanner systems can be implemented using a mechanically rotated gantry, or more recently, using a scanning electron beam. Either system may be operated in a conventional mode (e.g., non-helical) or, more recently, in a helical mode. Mechanically rotated gantry CT systems and their helical mode operation are described in U.S. Pat. No. 4,630,202 (Mori), issued Dec. 16, 1986. Scanning electron beam CT systems and their conventional mode operation are described in U.S. Pat. No. 4,352,021 (Boyd, et al.), issued Sep. 28, 1982, and in U.S. Pat. No. 4,521,900 (Rand), issued Jun. 4, 1985. Applicant refers to and incorporates by reference herein said patents to Mori, Boyd, et al. and Rand.
While the present invention is primarily directed to real time averaging of data acquired during helical mode operation of a scanning electron beam CT system, it is useful to review the problems attendant first with conventional mode operation of both types of CT systems, and then with helical mode operations.
In a mechanically rotating gantry CT system, an X-ray tube and an X-ray detector are located diametrically opposite from each other within a large doughnut shaped rotatable gantry. The patient or object to be X-rayed is positioned within the gantry opening and the gantry is mechanically rotated about the patient's lateral axis to scan an image slice. The X-ray detector signals are computer processed to produce images of the X-rayed patient. In conventional mode operation, the patient is stationary and a typically 360.degree. gantry scan is made, after which the patient is moved laterally a few mm and held stationary while a second scan is made, and so forth. Typically each lateral repositioning of the patient takes about one second, and each 360.degree. mechanical rotation of the gantry takes another second. The patient is moved laterally between scans a distance typically corresponding to the acquired image slice width (e.g., typically about 1 mm to about 10 mm). In conventional mode operation, the image slice width capability of the system (whether mechanical gantry or scanning electron beam) is a function of the system geometry and is not determined by the patient's lateral movement.
In a mechanically rotating gantry CT system operating in helical mode, the patient moves continuously laterally while scans are made at the rate of one scan/second. Because there is no one second dead time while the patient is laterally repositioned, scanning in helical mode is clearly faster than conventional mode. In helical mode operation, the image slice width is determined by the patient's lateral movement.
Briefly, mechanical gantry scanners have several intrinsic limitations, regardless of their mode of operation. First, because scanning is accomplished by mechanically rotating the gantry, scanning is intrinsically slow, requiring about one second/rotation. Second, heat dissipation associated with the scanning process occurs within the X-ray tube itself, and X-ray current is limited to about 125 mA. Finally, after about 30 seconds of continuous scanning, the X-ray tube must be turned off and allowed to cool, a severe limitation where multiple scan cycles are required to scan a large volume of a subject.
In a scanning electron beam system, such as described in U.S. Pat. No. 4,521,900 to Rand, an electron beam is produced by an electron gun and is accelerated downstream along the Z-axis of an evacuated chamber. Further downstream a beam optical system deflects the electron beam into a scanning path, typically about 210.degree.. The deflected beam is then focussed upon a suitable target, typically a large arc of tungsten material, which produces a fan beam of X-rays. The X-rays penetrate an object (e.g., a patient) and are then detected and computer processed to produce an X-ray image of a slice of the object.
In conventional mode use, lateral positioning of the patient takes about one second, after which a typical 210.degree. scan is accomplished in about 0.05 seconds or about 0.10 seconds, as selected by the X-ray practitioner operating the system. This rapid scan time results because the electron beam is scanned by magnetic deflection, rather than by mechanical rotation.
In brief, a scanning electron beam system has several intrinsic advantages over a mechanical gantry system. Not only is the scan time considerably faster (e.g., 0.05 seconds to 0.10 seconds compared to about 1 second), but because heat dissipation occurs in the large target ring, a relatively high beam current can be used (e.g., about 1,000 mA compared to about 125 mA). Finally, a scanning electron beam system can be continuously operated for about one minute before scanning must be interrupted to permit cooling.
In any X-ray system, a relatively rapid scan repetition rate is preferred providing certain minimum X-ray dosage requirements are met. Often the patient ingests or is injected with an iodine medium to enhance contrast in the X-ray image to be produced. A relatively slow scan repetition rate means a large dose of medium must be used so that some medium will remain active near the end of the scanning session. Unfortunately the medium is expensive and, in large dosage, unpleasantly invasive to the patient. Further, a slow scanning repetition rate means image quality may be impaired by organ functions. Overall, a slow scan repetition rate means fewer patients can be examined within a given time, thus representing an inefficient use of the CT X-ray facility.
The X-ray current or electron beam current associated with a system can also be an important system limitation because the object being examined will attenuate or absorb the X-rays. As is well known to those skilled in the art of imaging physics, there must be at least a minimum X-ray dose to successfully image a particular object with minimal photon noise. A successful image slice of an object requires an X-ray dosage defined as the product of the X-ray or electron beam current multiplied by the X-ray exposure time duration, commonly referred to as milliampere-seconds, or "mAs". For example, to image a human skull requires an X-ray dosage of about 800 mAs/slice, a human abdomen typically requires about 250 mAs/slice, and a human chest typically requires about 150 mAs/slice.
Consider for example a scan of a human abdomen with a mechanical gantry system in helical mode. Because the mechanical gantry system is limited to 125 mA X-ray current, the requisite 250 mAs dosage to acquire data for an adequate image slice of the abdomen is equivalent to a 125 mA X-ray current for an exposure of two seconds, e.g., 2 seconds per slice of about 10 mm, a "pitch" of 2:1. Thus in two seconds the gantry would make two 360.degree. rotations and would acquire a single 10 mm slice. But to be clinically useful, the overall image series comprising the X-ray examination should encompass about 40 cm of the patient's abdomen. However acquiring a 40 cm image series at a rate of 10 mm in two seconds would take 80 seconds, far too long to continuously operate a mechanical gantry scanner without severe X-ray tube overheating. With a 30 second maximum continuous on time before overheating, a mechanical gantry system could make only 30 scans (e.g., one 360.degree. scan/sec.) and acquire only about (30 sec./80 sec.).times.40 cm.apprxeq.15 cm of the abdomen, an insufficient span to be clinically useful.
In an attempt to make the best of a bad situation, a mechanical gantry CT scanner in helical mode is forced to move the patient laterally at approximately one slice width per 360.degree. revolution of the gantry. Unfortunately, the reconstructed image will exhibit relatively poor signal to noise characteristics because the requisite 250 mAs is not met (e.g., each one second slice is the product of a one-second scan of 125 mA X-ray current, only 50% of the requisite 250 mAs for an abdomen image). Further, one 360.degree. scan per each 10 mm slice represents a rather coarse pitch of 1:1, the pitch depicted in FIG. 2 in U.S. Pat. No. 4,630,202 to Mori. In addition, the mechanical gantry system operating in helical mode could only image 30 cm, whereas a 40 cm examination is typically required to be clinically useful.
This 1:1 coarse pitch gives rise to problems in reconstructing a CT image at each slice location. As is known to those skilled in the image reconstruction art, CT reconstruction assumes that the data to be processed are co-planar. However for image planes located between "turns" of the coarse 1:1 helix, actual data do not exist and must be interpolated from adjacent data points. Not only does this give rise to errors including artifacts in the images, but rather sophisticated data reconstruction techniques are required. In addition, the resultant rather poor quality image still represents a 30 second exposure, during which time there is contrast agent dissipation and body organ movement, further degrading utility of the image.
It is apparent from the foregoing that while helical mode scanning is considerably faster than conventional mode imaging, helical mode operation by a conventional mechanical gantry scanner has many deficiencies. As is set forth herein at the Detailed Description of the Invention, a scanning electron beam CT system can be operated in a helical mode to provide considerably faster imaging with a narrower or tighter pitch than is possible with a mechanical gantry system. However the narrow pitch and fast scanning repetition rate provided by such a system provide a faster stream of image data than can be economically stored in real time for later retrieval and processing.
There is a need for a real time method of processing or averaging data pertaining to an image slice provided by such a system. Preferably such a method should compress the slice image data to allow a smaller amount of compressed data to be stored and/or simultaneously processed to yield useful images. The present invention provides such a method. While directed primarily at helical mode scanning electron beam systems, the present invention may also be used advantageously with the slower mechanical gantry systems.